Makefile: use git describe for $COMMIT

Use "git describe --dirty --long" instead of "git rev-parse". As a
result, the commit ID will contain the closest tag, the number of commits
since the tag, and the (abbreviated) git commit sha (see example below).

NOTE that this tag is still unique and can be used instead of bare sha
for all git commands.

Example output of "runc -v | grep commit".

Before:
 commit: 4d87573

After:
 commit: v1.0.0-rc95-9-g6f55d074

This means that
 - the closest tag is v1.0.0-rc95
 - there were 9 commits after the tag
 - the abbreviated sha is 6f55d074

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Makefile

@@ -10,8 +10,7 @@ GIT_BRANCH_CLEAN := $(shell echo $(GIT_BRANCH) | sed -e "s/[^[:alnum:]]/-/g")
 RUNC_IMAGE := runc_dev$(if $(GIT_BRANCH_CLEAN),:$(GIT_BRANCH_CLEAN))
 PROJECT := github.com/opencontainers/runc
 BUILDTAGS ?= seccomp
-COMMIT_NO := $(shell git rev-parse HEAD 2> /dev/null || true)
-COMMIT ?= $(if $(shell git status --porcelain --untracked-files=no),"$(COMMIT_NO)-dirty","$(COMMIT_NO)")
+COMMIT ?= $(shell git describe --dirty --long --always)
 VERSION := $(shell cat ./VERSION)
 
 # TODO: rm -mod=vendor once go 1.13 is unsupported